1. Field of the Invention
The present invention relates to a protective layer transfer sheet.
2. Description of Related Art
A sublimation thermal transfer method has been widely used for forming thermal transfer images since it can easily provide high-quality images of excellent transparency, and high reproducibility and high gradation of neutral tints, which are equivalent to the conventional full-color photographic images. The sublimation thermal transfer method is a method in which a thermal transfer sheet that has a dye layer provided on one surface of a substrate and a thermal transfer receiving sheet that has a receiving layer provided on one surface of another substrate or an intermediate transfer medium that has a receiving layer provided on one surface of another substrate so as to be peelable from the another substrate are used in combination to form a thermal transfer image by applying heat from the back face of the thermal transfer sheet to displace color materials contained in the dye layer onto the receiving layer (for example, Patent Literature 1 and Patent Literature 2). Incidentally, with respect to the thermal transfer receiving sheet, a printed article is obtained by forming a thermally transferred image on the receiving layer, and with respect to the intermediate transfer medium, a printed article is obtained by forming a thermally transferred image on the receiving layer followed by transferring the receiving layer on which the thermally transferred image is formed onto an optional transfer receiving article.
Incidentally, although a thermally transferred image formed on the receiving layer by the sublimation thermal transfer method excels in gradation, the receiving layer on which the thermally transferred image has been formed is located on the outermost surface of the printed article, and thus, there is a problem of inferior durability of the thermally transferred image.
In view of the above mentioned circumstances, attempts have been widely made to obtain a printed article in which a protective layer is further provided on a receiving layer by transferring the protective layer on the receiving layer on which a thermally transferred image has been formed using a protective layer transfer sheet that has a substrate and a protective layer peelably provided on the substrate (may be referred to as a transfer layer) (for example, Patent Literature 3). According to the printed article on which the protective layer has been transferred, since the protective layer has been provided on the receiving layer, it is possible to improve the durability of the thermally transferred image formed on the receiving layer.
Recently, there have been many needs to obtain not only printed articles having a glossy appearance, but also printed articles having matte and semi-glossy appearances. It is possible to obtain such printed articles having glossy, matte, or semi-glossy appearance by adjusting thermal energy applied when the protective layer of a protective layer transfer sheet is transferred onto a receiving layer on which a thermally transferred image has been formed. It is possible to obtain a matte printed article, for example, by increasing thermal energy applied upon transfer of the protective layer, concretely, by applying thermal energy which causes cohesive failure in the binder resin contained in the protective layer upon thermal transfer of the protective layer. Meanwhile, it is possible to obtain a printed article having a glossy appearance by reducing thermal energy applied upon transfer of the protective layer, concretely, by applying thermal energy which does not cause cohesive failure in the binder resin contained in the protective layer on thermal transfer of the protective layer. In addition, it is possible to obtain a printed article in which a matte tone and a glossy tone are mixed, that is, a printed article having a semi-glossy appearance, by changing thermal energy applied on transfer of the protective layer on the region basis.
Incidentally, in the case of transferring a protective layer on a receiving layer on which a thermally transferred image has been formed using a protective layer transfer sheet, transfer is conducted with the receiving layer and the protective layer in close contact. If releasability of the protective layer is low, the receiving layer on which a thermally transferred image has been formed fuses with the protective layer on transfer of the protective layer, and the protective layer transfer sheet may break in the protective layer portion, or peeling traces may occur on the protective layer on peeling of the protective layer from the receiving layer. This leads to a problem of deteriorated print quality of a printed article after transfer of the protective layer. In particular, to obtain a printed article having a matte appearance, it is necessary to apply extremely high thermal energy, and thus, problems of the above fusion and peeling traces are more likely to occur.
Further, a printed article having a protective layer provided on the receiving layer is generally formed by superposing the protective layer of the protective layer transfer sheet on the receiving layer of the thermal transfer receiving sheet (the receiving layer of the above mentioned printed article) followed by applying thermal energy on the back face of the protective layer transfer sheet with heating devices such as the thermal head possessed by a printer to thereby transfer the protective layer on the receiving layer. Then, the printed article after the protective layer has been provided on the receiving layer is discharged through the conveying route in the printer to the discharge port. Here, in the case that a low scratch-resistant protective layer has been transferred on the receiving layer, there is a problem of impairing an aesthetic appearance of a printed article while the article is discharged through the conveying route to the discharge port due to various factors, such as contact with a conveying roller, which leads to scratches on the protective layer located on the outermost surface of the printed article. Conveying mechanisms of many printers are likely to scratch the protective layer. The need is high for a protective layer transfer sheet that has an excellently scratch-resistant protective layer.
Various studies of the protective layer transfer sheet having an excellently scratch-resistant protective layer have been made. For example, in the Patent Literature 4, a protective layer transfer sheet which has a protective layer comprising a composition formed by crosslinking a polyester resin with isocyanate has been proposed. However, the protective layer transfer sheet having a protective layer which has been proposed in the corresponding literature or protective layer transfer sheets having a protective layer which have been proposed so far have a certain effect on the scratch resistance, but it cannot be said that the scratch resistance under a severer circumstance is not perfect, and there is a room for improvement.